A. Field of the Invention
This invention relates to a new and useful method for liquifying natural gas. In particular, this invention relates to a method for producing liquid natural gas (LNG) having a high methane purity, which is well suited for integration with cryogenic gas processing plants used to recover natural gas liquids (NGLs).
Natural gas that is recovered from petroleum reservoirs is normally comprised mostly of methane. Depending on the formation from which the natural gas is recovered, the gas will usually also contain varying amounts of hydrocarbons heavier than methane such as ethane, propane, butanes, and pentanes as well as some aromatic hydrocarbons. Natural gas may also contain non-hydrocarbons, such as water, nitrogen, carbon dioxide, sulfur compounds, hydrogen sulfide, and the like.
It is desirable to liquify natural gas for a number of reasons: natural gas can be stored more readily as a liquid than in the gaseous form, because it occupies a smaller volume and does not need to be stored at high pressures; LNG can be transported in liquid form by transport trailers or rail cars; and stored LNG can be revaporized and introduced into a pipeline network for use during peak demand periods.
LNG which has been highly purified (i.e. about 95 to 99 mol % methane purity) is suitable for use as vehicular fuel, since it is clean burning, costs significantly less than petroleum or other clean fuels, provides almost the same travel range between fill-ups as gasoline or diesel, and requires the same fill-up time. High methane purity LNG can also be economically converted into compressed natural gas (CNG), another clean, economical vehicle fuel. The need for economical, clean-burning fuels such as LNG is particularly urgent because the Clean Air Act Amendment (CAAA) and the Energy Policy Act of 1992 are forcing companies with large vehicle fleets operating in areas with ozone problems, railroads, and some stationery unit operators to convert to cleaner burning fuels.
B. The Background Art
A number of methods are known for liquifying natural gas (consisting mainly of methane with minor concentration of ethane and heavier hydrocarbons). These methods generally include steps in which the gas is compressed, cooled, condensed, and expanded. Cooling and condensing can be accomplished by heat exchange with several refrigerant fluids having successively lower boiling points ("Cascade System"), for example as described in Haak (U.S. Pat. No. 4,566,459) and Maher et al. (U.S. Pat. No. 3,195,316). Alternatively, a single refrigerant may be used at several different pressures to provide several temperature levels. A single refrigerant fluid which contains several refrigerant components ("Multi-Component System") may also be used. A typical combination of refrigerants is propane, ethylene and methane. Nitrogen is sometimes used as well. Swenson (U.S. Pat. No. 4,033,735), Garier et al. (U.S. Pat. No. 4,274,849), Caetani et al., (U.S. Pat. No. 4,339,253), and Paradowski et al. (U.S. Pat. No. 4,539,028) describe variants of the Multi-Component refrigeration approach. Expansion is generally isenthalpic (via a throttling device such as a Joule-Thomson valve) or isentropic (occurring in a work-producing expansion turbine).
Despite the availability of these methods, there are very few facilities in the United States that can produce significant amounts of vehicular grade LNG. In principle, any of the above methods can be used to liquify natural gas. However, the capital cost of constructing and maintaining refrigeration systems for producing LNG can be high. Auxiliary refrigeration systems have high energy expenses, using considerable amounts of fuel gas or electricity and producing significant air emissions (if fuel gas is used).
The various existing LNG production processes and possibility of producing LNG at various types of natural gas processing plants will now be considered. It will be seen that there remains a need for an economical liquifaction process which is compatible with commonly available types of natural gas processing plants and which makes it feasible to produce LNG in the large volumes and with the high purity which would be necessary for it to be practical as a vehicle fuel (see also "LNG Supply", LNG Express, Volume IV, No 1, pp. 1-4, January 1994, for further discussion of the need for increased vehicle grade LNG production in the U.S., possible methods for producing LNG, and the desirability of modifying existing plants to produce LNG).
LNG Peak Shaving Plants are used to liquify natural gas which is stored for later use during peak demand periods, to insure that municipal gas distribution grids have adequate gas supplies during severely cold weather. These plants typically utilize cascade or multi-component refrigeration systems to liquify pipeline quality gas. LNG Peak Shaving Plants produce the majority of LNG in the U.S., but only a fraction of their capacity is available for transportation use. Furthermore, most peak shavers do not produce an LNG product with a high enough methane content to be used as a vehicle fuel. LNG Peak Shavers usually liquefy pipeline quality gas which typically contains too much ethane and heavier hydrocarbons to make a vehicle grade LNG product.
Pachaly (U.S. Pat. No. 3,724,226) describes a plant which combines cryogenic fractionation with an expander cycle refrigeration process to produce LNG. The intended purpose of this plant is the liquifaction of natural gas at remote locations in order to facilitate transportation. This plant does not, however, produce high methane-purity LNG and furthermore the design is such that operating costs will be high.
"Grass Roots" or dedicated LNG plants are new plants designed and installed specifically for the purpose of producing vehicle grade LNG. These plants may have various designs, but all tend to use auxilliary refrigeration systems like those described above. The main disadvantage of this type of plant is that installing a new facility is more expensive than modifying an existing facility.
Nitrogen Rejection Units (NRUs) utilize cryogenic fractionation to liquify methane and separate it from gaseous nitrogen. NRUs are used at sites where the natural gas has a high nitrogen content, either naturally occurring or because nitrogen was injected into the petroleum reservoir to maintain reservoir pressure and increase the recovery of oil and/or gas. The methane purity of the LNG produced at these plants is often sufficiently high for use as a vehicle fuel. However, there are not a large number of these sites and they are often in remote areas, so NRUs do not represent a major source of LNG in the United States. In addition, they require the use of a large amount of auxiliary refrigeration.
Another type of plant which processes natural gas is the natural gas liquid (NGL) plant, which is used to recover NGLs. NGL recovery comprises liquifying and separating the heavier hydrocarbon components of natural gas (ethane, propane, butanes, gasolines, etc.) from the primarily methane fraction which remains in gaseous form (residue gas). The heavier hydrocarbons are worth more commercially as liquids than as natural gas. NGLs are sold as petrochemical feedstocks, gasoline blending components, and fuel. These plants also typically remove non-hydrocarbons such as water and carbon dioxide to meet gas pipeline restrictions on these components. There are hundreds of such NGL plans throughout the U.S. NGL plants include lean oil absorption plants, refrigeration plants, and cryogenic plants. To the best of the inventors knowledge, such plants are not presently used to produce LNG (liquid natural gas). However, if a cost effective process for liquifying the residue natural gas could be integrated with these plants, NGL gas processing plants could become a significant source of vehicle fuel in the U.S.
Existing LNG Peak Shavers, NRUs and natural gas processing plants used to recover NGLs may be modified to produce vehicular grade LNG fuel by the addition of fractionation systems and auxiliary refrigeration systems. Additional cryogenic distillation systems may be used to increase the LNG purity by removing ethane and heavier hydrocarbons from natural gas in order to produce fuel quality LNG. However, since installation of fractionators and auxiliary refrigeration systems is very expensive, this is not always an economically feasible approach for producing high-purity LNG suitable for vehicle fuel.
We have discovered a novel manner in which a basic cryogenic NGL plant design can be modified to make a plant for producing high methane purity LNG without the need for additional fractionation and refrigeration systems.